The present invention relates to security documents including a security image composed of a printed security image and a printed complementary security image. The security image is arranged to provide an indication of document authenticity.
Conventional security documents comprise a security image including security image elements designed such that an attempted duplication or reproduction results in the formation of a readily apparent warning image on the face of the duplicate document. The security image elements are arranged such that the presence of the warning image is not readily apparent on the original. Examples of security documents of this type are illustrated in U.S. Pat. Nos. 4,579,370, 5,149,140, 5,197,765, 5,340,159, the disclosures of which are incorporated herein by reference.
As is clearly illustrated in these prior art references, security image elements typically comprise background elements and message elements. The respective tones of the background elements and the message elements are selected such that the background elements and the message elements are substantially indistinguishable from one another to the naked eye. These respective tones combine to define an overall security image element tone. Further, the background elements and the message elements are selected such that either the background elements or the message elements are not readily duplicated by a color copier at a particular copier setting. In this manner, upon copying of the security document, a visually perceptible message defined by the message elements is produced on a copy of the document. The message may be one or more images, symbols, words, etc.
Some conventional security documents are printed in two or more colors that are blended to achieve a smooth transition from one color to the next. This is commonly achieved through the use of a split fountain, i.e., a print unit with special dividers which separate one color from another. The split fountain technique is, however, troublesome in some instances because it is typically difficult to control print quality and requires frequent production stoppages for cleaning and other related maintenance.
Accordingly, there is a need for a security document production scheme that enables efficient creation of a security document including a multi-color security image where the colors blend together in an aesthetically pleasing manner. Further, there is a need for a multi-color security document production scheme that avoids conventional difficulties related to printing multi-color documents.